I find 30/30’s Billat style to be very beneficial. I also find them easier mentally compared to longer intervals. As you adapt you could do additional intervals extending time in zone instead of raising intensity.
There is a thread on this topic in the forum that may be worth a read:
https://www.trainerroad.com/forum/t/veronique-billat-training-model
You can also Google “Veronique Billat 30 30” for more info.